This invention relates generally to the art of plastic film forming extrusion machines and particularly to the maintenance of a constant thickness and density of plastic material being extruded on to a casting drum and the like.
Numerous devices have been developed in an attempt to assure a constant thickness of material emerging from extruders. Most utilize gaging systems to measure the compressed material at some point up-stream from the extrusion rollers. Examples of such devices are disclosed by Brand, et al. in U.S. Pat. No. 3,930,774 and Graves, et al. in U.S. Pat. Nos. 4,137,025 and 4,152,380. The obvious problem with such devices is that the compressed material must be measured as either too thick or too thin before any remedial action can be taken. Also, gaging methods are typically expensive to install and to maintain.
Greenberger, in U.S. Pat. No. 3,901,635 disclosed a control system for briquetters using a position measurement of one roll relative to the other to maintain a constant thickness of compressed material and a pressure measurement of one roll relative to the other to maintain a constant density of compressed material. But, the thickness and density of compressed material have never been calculated and adjusted with only one type of measurement and only one type of adjustment, an easier and more efficient process.